The present invention relates generally to optics, and more particularly, to an optical system and method for producing a compact uniform illumination beam for use in optical navigation devices, such as an optical mouse.
Mechanical mice are well known to computer users as a useful tool for positioning a cursor and selecting functions in different applications. These mechanical mice use a ball and related sensors to detect the position of the mice.
In recent years, there has been an increase in the number of manufacturers offering optical mice as an alternative to the mechanical mice because of the increased accuracy and durability of optical mice.
Optical mice use light to detect the position of the mice. Typically, an optical mouse has a light source, such as a light emitting diode (LED), for illuminating a surface, such as a mouse pad or surface of a table. The light strikes the surface and a portion of the light is reflected. The optical mouse has an on-board camera for recording the reflected light. Based on the reflected light, an electronic computation unit determines the position of the optical mouse.
It is important that the illumination beam is as compact and uniform as possible. Since the accuracy of the positional information derived from the reflected light is dependent on minimizing the amount of differences in the light (e.g., contrast) that are caused by the equipment so that the contrasts as closely as possible reflect the surface differences.
Unfortunately, the illumination source that is widely used is an LED die, which has a bond pad and associated wire in the middle of the die. The bond pad and wire cause the resulting image of the illumination beam to have an undesirable dark region in the center. FIG. 10 illustrates an exemplary LED die and the corresponding illumination beam that is produced thereby.
The prior art approaches to lessen the effect of this blind spot or dark region only offer tolerable solutions that each has associated shortcomings. In a first prior art approach, an output face of a prism element is used that has three plane facets at a small angle with respect to each other. This facet arrangement causes the formation of three images of the die that are laterally displaced so that the dark spot of one image falls on a bright bar of an adjacent image. For example, each image is offset in such a manner as to reduce the blind spot with another bright region in the image. Although the non-uniformity (i.e., the dark region or blind spot) in the triple image is less pronounced than before, the resulting image has a lower average luminance. In other words, the slight increase in uniformity is at the expense of the luminance of the illumination spot, which is less bright overall than before.
In a second prior art approach, a tapered light pipe shown in FIG. 11 is used to re-direct incoming light and reduce the dark spot. The light pipe is tapered so that the output end is smaller than the input end. The input end collects light flux from the LED. The size of the input is made to have a much larger dimension than the dimension required for the illuminated region.
Unfortunately, this approach offers a design very little control over the routing and angular distribution of the resulting light. The approach also has the following additional disadvantages. First, since the light pipe needs to be an angle relative to the surface to be illuminated, both the light source and light pipe require special mounting to meet these angle requirements, thereby increasing the manufacturing costs. Second, the light travels through the light pipe, the angle of the light beams increases every time the light beam is reflected off of one of the walls of the light pipe. Consequently, the output light beam has a high degree of divergence that results in a high angle of incidence with the surface of illumination. This high angle of incidence may cause certain contrasts resulting from certain types of surfaces to be washed out or negated by the illumination beams, thereby leading to poor performance of the optical mice on certain surfaces.
Based on the foregoing, there remains a need for an optical system and method for producing a compact and uniform illumination beam that overcomes the disadvantages set forth previously.
According to one aspect of the present invention, a method and system for producing a compact uniform illumination beam that does not have a positional blind spot are provided. First, non-collimated light beams that have a positional blind spot are received from a light source. Next, the non-collimated light beams are converted into approximately collimated light beams without the positional blind spot. Then, the collimated light beams are split into a plurality of split beams. These split beams are then overlapped to form a compact and uniform illumination beam.
In one embodiment, the method and system of producing an illumination beam involves receiving generally divergent light beams, which may be provided by a light emitting diode (LED) light source. Second, these divergent light beams are converted into generally convergent light beams by using a collimating unit, such as a collimating surface, lens, or mirror. Next, the convergent light beams are split and overlapped in one or more directions. For example, in the preferred embodiment, the convergent beams are first split and overlapped in the horizontal direction and then split and overlapped in the vertical direction. The convergent light beams are split and directed and overlapped to form overlapped collimated light beams that produce a compact, uniform illumination area on the illumination surface.